The present disclosure relates generally to LED lighting systems and LED control methods therefor.
There are different kinds of lighting devices developed in addition to the familiar incandescent light bulb, such as halogen lights, florescent lights and LED (light emitting diode) lights. LED lights have several advantages. For example, LEDs have been developed to have lifespan up to 50,000 hours, about 50 times as long as a 60-watt incandescent bulb. This long lifespan makes LED light bulbs suitable in places where changing bulbs is difficult or expensive (e.g., hard-to-reach places, such as the exterior of buildings). Furthermore, an LED requires minute amount of electricity, having luminous efficacy about 10 times higher than an incandescent bulb and 2 times higher than a florescent light. As power consumption and conversion efficiency are big concerns in the art, LED lights are expected to replace several kinds of lighting fixtures in the long run.
A LED is a current-driven device. As commonly known in the art, the brightness of a LED is substantially dominated by its driving current, and the voltage drop across the LED illuminating is about a constant. Accordingly, a driver for driving LEDs is commonly designed to function as a constant current source or a controllable current source. FIG. 1 shows LED lighting system 10 according to U.S. Pat. No. 6,989,807 in the art. LED string 14, comprising LEDs 15a, 15b, and 15c, connected in series, is coupled to a power source provided by bridge rectifier 12, which is connected to a branch circuit providing AC voltage VAC. LED controller 16 detects input voltage VIN output from bridge rectifier 12 and accordingly controls current sources 18a, 18b and 18c. As taught in U.S. Pat. No. 6,989,807, input voltage VIN is sensed for determining how many LEDs in LED string 14 are excluded from being driven. In some instants, for example, the most downstream LED 15c is not driven because current source 18c is turned off. FIGS. 2A and 2B demonstrate two different luminance intensity results from LED lighting system 10 driven by branch circuits of 200 ACV and 100 ACV, respectively. In FIGS. 2A and 2B, threshold voltages VTH1, VTH2 and VTH3 are the minimum voltages required for turning on the LED string with only LED 15a, the LED string with LEDs 15a and 15b, and the LED string with LEDs 15a, 15b and 15c, respectively. As VIN gradually increases over threshold voltages VTH1, VTH2 and VTH3, LEDs 15a, 15b, and 15c are sequentially turned on, and vice versa. Each LED in FIG. 1 is intended to be driven by a fix current when it shines. Thus, the present number of the LEDs joining to shine decides the instant luminance intensity of LED lighting system 10. The top boundaries of the shadowed areas in FIGS. 2A and 2B represent luminance intensity of LED lighting system 10.
Nevertheless, LED lighting system 10 shines brighter in FIG. 2A than it does in FIG. 2B, because the shadowed area in FIG. 2A, roughly corresponding to the average luminance intensity of LED lighting system 10, is larger than that in FIG. 2B. Taking LED 15a for example, it is turned on earlier but turned off later in FIG. 2A than it is in FIG. 2B. So are LEDs 15b and 15c. The higher input voltage VIN, the longer turn-on time of each LED in LED string 14, and the brighter LED lighting system 10. A LED lighting system with a constant average luminance intensity that does not vary along with the AC voltage of a branch circuit is much more preferred, nevertheless.